The present invention relates to an adjustable resistor comprising a strip of resistance material arranged on an insulating carrier and a slider made of conductive material movable along the strip and contacting the strip.
Such adjustable resistors are generally known, for instance in the form of potentiometers, wherein the slider is formed by an element usually made of copper or silver which is movable along a carbon strip. Such adjustable resistors are also known in the form of for instance ceramic printed circuit boards on which strips of resistance material are arranged along which a slider made of copper or silver is movable and which can be used in particular to adjust the speed of an electrical hand-tool. In both cases a slider of a rigid, though slightly resilient material is moved along a resistance strip. The resilience of the material is necessary herein to press a contact surface of the slider against the resistance strip.
It is noted here that while such a material is resilient, it is hardly flexible. This has the consequence that generally only one point of the slider is in contact with the resistance strip. A current can thus flow only at this one point between the slider and the resistance strip. The current flowing through the resistance strip to the slider will, starting from one end of the resistance strip, initially be fully distributed over the width of the resistance strip, but in the vicinity of the contact position of the slider will be distributed with a greater current density to this contact position. This results in an uneven current distribution and thus to a locally higher current density and a locally greater resistance. The curve of the adjustable resistor is thus not linear. This problem otherwise occurs both when the adjustable resistor is used purely as an adjustable resistor and when it is used as a potentiometer. In the case of a potentiometer, a current flowing from the one end of the resistance strip to the other end of the resistance strip is superimposed on the said current.
U.S. Pat. No. 4,833,440 provides an adjustable resistor comprising a strip of resistance material arranged on an insulating carrier and a slider made of conductive material guidable along the strip and contacting the strip, wherein the slider is manufactured from elastomeric material.
This prior art resistor avoids the problems mentioned above.
However, this prior art resistor has the draw back that an uneven distribution of the current is created through the cross-section of the resistance strip in the vincinity of the slider, when the contact surface between the resistance strip and the slider does not extend over the resistance strip.
The present invention aims to provide such a resistor, wherein this problem is avoided.
This aim is reached in that the resistance strip is divided into two parallel extending strips of equal width, and that the contact surface of the slider is interrupted and that each of the thus formed parts of the contact surface makes contact with one of the strips.
The problems associated with the prior art are avoided by the configuration of the two resistance strips. The resistance cannot become smaller between the two sliders than the resistance of the fixed connection there between.
A slider manufactured from an elastomeric material will, as a result of its flexibility, make contact with the resistance strip over a relatively large part of the width of the resistance strip, so that the problems of an uneven current distribution associated with one-point contact are avoided.
The use of an elastomeric material further provides the option of dimensioning and shaping the slider such that it can be handled easily by means of a positioning machine, and the problems of the usually elongate rigid sliders on the production line are avoided.
The slider is preferably manufactured from conductive rubber or a conductive plastic. According to yet another embodiment the slider is clamped in a carriage movable parallel to the strip.